The European Medicines Agency (EMA) has also started reviewing data on Pfizer's antiviral treatment, Paxlovid, as an oral treatment for Covid-19, with a full rolling review expected to begin next week.
It said Merck’s antiviral pill, known as molnupiravir, can be used for at-home treatment for adults who have been infected but do not yet need extra oxygen and who are at increased risk of developing severe disease.
National authorities may decide to use the pill in emergency settings “in light of rising rates of infection and deaths due to Covid-19 across the EU”, the regulator said.
The UK was the first country to approve Merck’s drug, developed together with Ridgeback Biotherapeutics. Merck has also sought emergency use authorisation in the US.
"EMA issued this advice to support national authorities who may decide on possible early use of the medicine prior to marketing authorisation, for example in emergency use settings, in light of rising rates of infection and deaths due to Covid-19 across the EU," the agency said.
The Merck pill should not be used by pregnant women or women who could become pregnant, the EMA said.
"These recommendations are given as laboratory studies in animals have shown that high doses of [the Merck pill] can impact the growth and development of the foetus," it added.
Austria imposed a nationwide lockdown on Friday and German authorities said they have not ruled out a similar move, as cases rise and intensive care facilities fill up.
The Merck and Pfizer pills represent a potentially groundbreaking step in the fight against Covid-19 as studies show they decrease the risk of hospital admission and death in high-risk patients.
The EMA said it wanted to be able to help national authorities combat the latest wave "in the shortest possible timeframe".
The Pfizer pill belongs to a class of antivirals called "protease inhibitors", which block the action of an enzyme that is critical to viral replication.